INFLUENCE OF INBREEDING ON HYDATINA SENTA. II 



that the constitution of the F! individuals is Aim that mal.< - 

 them vigorous, but the interaction of a nucleus 1 of constitution 

 Mm with a mass of cytoplasm accustomed, so to speak, to a 

 nucleus of constitution AIM or mm. If it were possible to remove 

 from an egg its own nucleus, and substitute for it a nuclru 

 slightly different, but not so different as to be "incompatible," 

 with a diploid set of chromosomes, and have it develop normally, 

 it should, on my view, produce an individual more vigorous than 

 its parent, even if the introduced nucleus were complch ly 

 homozygous. On this view, a line that has become homozygous 

 need not have reached its minimum of vigor, as it must on both 

 of the other views discussed. 



In animals that reproduce by parthenogenesis or fission, the 

 long continued interaction between cytoplasm and nuclei that 

 suffer no change of genotypic constitution, may bring about an 

 approach to equilibrium, thereby decreasing metabolism, and 

 hence vigor. 



In like manner, continued production of somatic cells without 

 change of genotypic constitution in the nucleus may cause an 

 approach to equilibrium resulting in senescence in the metazoan 

 individual. The cases in which a high standard of vigor is 

 maintained notwithstanding inbreeding, as in wheat and tobacco, 

 or in the absence of genotypic change, as in Woodruff's para- 

 mecia, are not so easily explained. They may be due to any one 

 of several causes. If metabolism be maintained by a reversible 

 reaction between nucleus and cytoplasm, vigor could be sustained 

 indefinitely. Or the interaction may be kept up by changes in 

 the cytoplasm, changes due to variable nutrition or other external 

 agents. These are mere suggestions. 



East and Hayes have suggested a physiological foundation for 

 the heterozygosis view. They hold that increased vigor in 

 hybrids is due to more rapid cell division, and that the stimulus 

 to this more rapid division is given by the presence of genes in 

 the heterozygous condition. To me it seems that the stimulus 

 is due, not to any effect that the two parental contributions to 

 the nucleus may have directly upon one another, but to the effect 



1 It is assumed without argument that the representatives of body characters 

 reside in the chromosomes. 



